Member postings for S K

OK, got it! Thanks.

I believe this means that the optimum place to apply an impulse is at the center of percussion. Neat!

If you imagine the object is simply a uniform rod, floating weightless in space, and you hit it in the center (that would be the center of percussion in this case), then the entire rod will move in the direction of the force, including an imaginary support at one end.

It's not the case that the support will not feel a force. If it didn't feel any force, then that end would not move by definition, regardless if it's held or left free. You may mean something similar, like there's no rotational torque. No? (I'm not claiming knowledge here, I'm inquiring.)

Duffer:

I've watched your project since you began posting with great interest.

To be honest, I did not feel that using a rigidly-attached thin carbon rod as a pendulum would likely work out well, but it's been very interesting to see the progress, especially your detailed data analysis. I wish I could figure out why you have those strange discontinuities in timing. It's got to be something simple and basic - one of those slap-your-forehead moments.

For a light pendulum, a very thin spring (e.g. 0.002" maybe 1/8 to 1/4" in width, and a spring length of maybe 1/8 inch is in the ballpark as recommended in that book.

Carbon fiber is indeed capable of absorbing moisture. If it's "raw", you might think about putting a thin wipe-on polyurethane coating on it.

I also think you should consider switching to the Sharp. It will be much better, and it's dead easy to use with an Arduino!

Anyway, I shouldn't hijack this thread too much. I can comment more in your thread.

Edited By S K on 24/01/2023 20:19:39

Edited By S K on 24/01/2023 20:31:53

Where did you get the thin BeCu strip? I've been looking for some of that!

You may find the book "Accurate Clock Pendulums" by Robert James Matthys interesting. I think some of the material in it, such as suggested spring dimensions, is more anecdotal than rigorously scientific, but he basically claims "the shorter the better" to minimize undesired movement, e.g. wobbling at the bottom of the spring due to the impulse's effect on the top of the rod. But anyway, there's a lot of excellent information in it.

Yes, I'm aware of the many pitfalls of using knives, but that's where I'm headed for now. I'm mostly interested in making a precision pendulum for the moment. In fact, I'm not even explicitly trying for a second's pendulum. And even pendulums mounted on a spring often use gimbals to insure that the orthogonal axis is perfectly level.

I did make a 3D printed set of knives and anvils for the pendulum just to test the concept. It worked surprisingly well despite the dull edge and high friction. Starting with maybe five degrees of swing, it rocked freely for a good half-hour or more. There was no perceptible walking, though I don't know how well it would work over days or months. Now I have to test my extremely limited machining skills, on extremely limited equipment, to make a proper version. But if I had some nice spring material I might abandon the knives after all.

If I did add time-keeping, I'd likely go with a purely electronic readout, or else three dials and three steppers rather than one stepper and a gear-train. I'd rate the speed of the clock mathematically rather than attempt to dial-in the pendulum's period.

Thanks again!

I looked at that Sharp photointerrupter.

At 10mm, it's the widest-gap one that Sharp makes. The detector aperture is rather large at 1.8 mm, though, and distinctly larger than most other options.

It has voltage regulation up to a 17V supply and a Schmitt trigger circuit, which are both nice. Its rise/fall time is quoted as distinctly faster than what I measured for mine, but it has a long-ish propagation delay. The latter should not matter as long as it's uniform.

I also looked at some larger-gap Omron and Lite-On ones. For example, the Omron EE-SX3070/-SX4070 is interesting. It has a 8mm gap, a 0.5mm detector aperture and a Schmitt trigger circuit, but no regulation and considerably slower rise/fall times.

Anyway, your Sharp one looks like a good over-all choice. I'll pick some up instead of continuing with my no-name versions.

Thanks!

It may be that the forces are so low that your spring will not be unduly affected. I presume yours is one or two pieces of thin flat spring steel or similar? It's a little hard to see in your dead-on photo. It sure looks solidly constructed, though!

What is the spring material, thickness, width and free length that you are using? It kind of looks to me like they are longer than necessary or optimum. I think a shorter spring is usually preferred, though I'm unsure of the scale in the photos.

For my "spring," I have been intending to use tool-steel knife-edges on some pieces of very flat borosilicate glass. The fact that the knives just pivot freely on glass is the reason I've felt so concerned about forces on the "spring." I can't afford for them to "walk" or bounce even slightly.

Thank you for the reply to my questions.

Thanks for the optical switch part number. At the moment, I'm using a cheap no-name switch with roughly 1 us rise/fall times. A part in 10^6-ish seems OK, but I'll see if that part can do better.

The slot approach reducing stray light makes sense, thanks. I've mocked up the use a pin just for low air resistance, but will reconsider.

Concerning the position of the coils, I can see your points, but it still bothers me. In a conventional pendulum clock, the impulse is most often somewhat near the top, either made by the escapement directly or through a fork a little lower around the shaft. So that's in line with your choice, at least.

In the Shortt-Synchronome, the impulse is applied roughly one-third down from the top of the shaft, but that may have been for convenience, e.g. at a position in which the swing is an appropriate width.

I guess my "bother" concerns the shaft and how stiff or flexible it is, and how the position of the impulse on a non-ideal shaft might influence performance. If I were to just naively guess where the optimum position might be, I'd guess either the bottom, the center of gravity, or the center of percussion. But I'm not sure which and haven't studied the math nor made a computer model.

The center of percussion doesn't feel right because the pendulum is not floating free in space, it's constrained at one end, and the goal is not to shift the whole pendulum horizontally. If impulsed there, an equal force on the hinge would seem to be applied. Therefore, I'd guess that below the COP would be better. The COG would probably be close to the COP, and also doesn't feel right for similar reasons. Therefore, I've naively presumed that as close to the bottom as practical would be best, if only because that would appear to apply the smallest force to the hinge (is this wrong?).

In a pendulum with a light carbon shaft and a very heavy bob (I'm jealous of your tungsten-alloy one!), both the COP and COG are likely to be very low anyway - maybe even within the bob in your case. My shaft is Invar and my bob is a brass weight that looks far lighter than yours, so that's not the case in my setup.

As Invar is only weakly magnetic (it's essentially a stainless steel), I was intending to use a steel collar a little above the weight, impulsed by an open solenoidal coil. Its position was to be just far enough above the bob for any air turbulence due to the coil to be low. I have not made the coil yet, but my thought is that the amount of energy needed to be imparted should be rather low, so the distance from the shaft to the coil may not be a difficult impediment to overcome.

It will take me a while to work through your explanation about the coils themselves.

Any further thoughts or explanations? I'll be watching this thread for sure!

Thank you again.

Hello John,

Excellent work! I'm building a somewhat similar project, and have several questions for you.

• Why are you using a Helmholtz coil configuration vs. say a one-sided solenoid? I get that it provides a more uniform magnetic field, but what are your thoughts about it?
• Why is it positioned almost at the top? Wouldn't that require far more force than nearer the bottom, and also possibly upset the nearby spring hinge unnecessarily?
• Why did you move from a pin to a slot for the optical switches?
• Which specific model optical switches you are using and why? (I'm looking for the fastest and hence hopefully highest time-resolution ones I can find.)

I have more, but these will do for now. Again, nice work!

Thank you.